memory leak tests

[david at tethera.net]

These are slightly rough around the edges, but I think they are useful.
They already helped me track down a memory leak in notmuch new
(id:1355234087-6886-1-git-send-email-david at tethera.net, 
id:1355196820-29734-1-git-send-email-david at tethera.net)

They also would have caught the restore leak Jani pointed out in
parse_sup_line, although that is a bit of hindsight since I didn't
write the obvious test until Jani pointed out the leak.